1. Field of the Invention
The present invention is related to input circuits, and more specifically to input circuits having hysteresis.
2. Background Art
There is a desire to design an input circuit with hysteresis to mitigate the effects of noise from an unregulated input signal received at the input (pad). It is advantageous for the input circuit to have tight control of the input switching point.
Furthermore, integrated circuit (ICs) are being built in reduced feature size technologies and reduced core voltage levels in the range of 1.2V. These ICs may need to interface with input signals with higher voltage levels.
The present invention includes an input circuit having hysteresis to mitigate the effects of input noise. The input circuit receives an analog input signal and determines whether the unregulated analog input signal is a high or a low voltage. The input circuit outputs a regulated low voltage (i.e., xe2x80x9c0xe2x80x9d) for a low input signal, and outputs a regulated high voltage (i.e., xe2x80x9c1xe2x80x9d) for a high input signal.
The input circuit includes a comparator that has a hysteresis property, where the output transition from low-to-high requires a higher input voltage than the transition from high-to-low. In other words, the comparator is configured to have two trigger voltages. In order for the output to transition from low-to-high, the input voltage must rise above a first threshold voltage. In order for the output to transition from high-to-low, the input voltage must fall below a second threshold voltage, where the first threshold voltage is higher than the second threshold voltage. The two separate thresholds help prevent noise on the input signal from inadvertently causing the input circuit to change state. Furthermore, the comparator includes a feedback path from an output of the comparator to an input of the comparator. The feedback path causes some delay in any output voltage transition (i.e. high-to-low output transition or low-to-high transition), which further enhances the hystersis effect and improves noise immunity.
An embodiment of the circuit interfaces with high voltage (e.g., 5V) input signals and outputs low voltage (e.g., 1.2V) output signals. In other words, the input circuit also provides a voltage transition while detecting the low-to-high and high-to-low transitions.